The present invention relates to vibration isolators and position control actuators. More particularly, the present invention pertains to a compact hybrid pneumatic-magnetic isolator-actuator capable of large stroke, near frictionless operation, and high ratio of actuation force to weight.
Passive isolation systems, composed of multiple passive vibration isolators, are commonly used for preventing vibration force input to a payload from a vibrating support base. Base motions of a frequency sufficiently higher than the break frequency of the isolation system (i.e. the natural frequency of the payload on the isolators) are significantly attenuated by the isolators and are prevented from passing into and disturbing the payload. Soft mounts (passive isolators) simply allow the payload to be held still in inertial space by its own inertia, but only at frequencies high enough that inertia forces on the payload are large compared to stiffness forces transmitted through the isolators, i.e. at frequencies above the system natural frequencies.
The static load position of the payload and the movement of the payload under low frequency dynamic loads, i.e. those loads having vibration frequency components below the break frequency, are dependent upon the spring stiffness of the isolators, the mass of the payload, and the low frequency acceleration forces being applied to the payload. When the body acceleration forces are constant, payload position relative to the base remains constant. When low frequency body acceleration forces upon a payload vary, such as would the forces on a payload in an aircraft undergoing various maneuvers, the payload position relative to the base varies. This is often detrimental to the function of the payload.
The nature of passive isolators is that they do not provide for force or position control that is necessary for payload stabilization when varying low frequency body forces are applied. Rather the force applied to the payload by a passive isolator is dependent upon the displacement of the payload relative to the support base. Passive isolation therefore always involves a compromise between position control accuracy and dynamic isolation.
To control the position of the payload relative to its base and maintain vibration isolation, an active isolator, i.e. an isolator-actuator is required. The isolator-actuator actively controls the low-frequency force applied to the payload at any and all positions of the payload relative to the base, i.e. regardless of the extension position of the support isolators.
Position actuators such as hydraulic actuators are commonly used for position control. Hydraulics can afford significantly high bandwidth position control because of the inherent incompressibility of the hydraulic fluid. However hydraulic actuators have several disadvantages. Position control and base motion isolation can be effected only up to the bandwidth limit. At the bandwidth limit, often set by dynamics of the payload, the actuators become stiff and unable to provide isolation to base motions of higher frequency. Secondly, high fluid pressures, which allow wide bandwidth performance, require tight sliding seals to prevent leakage. These seals inevitably introduce friction, which degrades high frequency isolation and introduces a deadband into low frequency position control. In effect, seal friction produces dynamic forces on the payload, thereby defeating the desired isolation function. Finally, hydraulic actuators are undesirable for use in vacuum because even very small oil leakage would cause contamination of any nearby equipment.
Pneumatic isolators with air bearing support have been used for providing low to zero friction payload support. However, a dilemma faced in the use of pneumatic isolators as actuators is that the compressibility of the air in the isolator-actuator and pneumatic control valve severely limits the control bandwidth.
Additional control problems arise from the control system components necessary to drive the actuators. Servo-valves used for either hydraulic or pneumatic control are commonly proportional spool valves, which always have a degree of friction and therefore produce hysteresis in the actuator control.
Many of the problems described above have been addressed in an earlier patent, U.S. Pat. No. 6,196,514 B1, by this inventor, which disclosed a Large Airborne Stabilization/Vibration Isolation System (AS/VIS). In that patent a payload is supported by an array of frictionless pneumatic isolators integrated with large, high-force electromagnetic voice coil actuators. The pneumatic isolators support the entire payload weight with very low stiffness and no friction. The voice coil actuators act in parallel with the isolators to effect position control with reasonably high bandwidth without degrading high-frequency isolation.
However the AS/VIS design has several limitations. The voice coil actuators are quite heavy, being required to control the position of a large payload. The passive elements connecting the payload to ground in vertical and horizontal directions are “nested”, meaning one rests upon the other and they act in series. While producing excellent isolation, the system is too fragile to meet the crash load requirements for an airborne system, and thus requires a parallel system of slack tethers in order to meet airworthiness requirements. Finally, the uncontrolled exhaust from the air bearings of the isolator-actuators render the system unsuitable for use in vacuum. The present invention advances the state of the art by addressing all three of these limitations.
Other devices of the prior art have offered damping and vibration isolation with active control of the isolator damping characteristics, but they do not provide both actuation with high force capacity and the isolation characteristics of very soft passive mounts. U.S. Pat. No. 6,003,849 discloses a hybrid isolator and structural control actuator strut. In this device communicating fluid reservoirs are used to provide payload motion damping and vibration isolation, and an active enhancement mechanism is used to alter the fluid pressure in the fluid reservoirs to modify the damping characteristics and add modest actuation capability to the device. This device fails to provide the large actuation force and stroke capability and low break frequency isolation needed for stabilization of large airborne optical systems. U.S. Pat. No. 6,129,185 similarly offers vibration damping and isolation while magnetically destiffening the support system to offer lower isolation frequencies, but it does not afford the combined actuation ability and high degree of isolation of the present invention.
Actuation capability is afforded in other devices of the prior art but without the high degree of isolation, and near frictionless operation of the present invention. U.S. Pat. No. 5,060,959 describes an electrically powered active suspension strut for a vehicle. This device incorporates a spring or fluid system for load support and therein fails to provide the very low frequency vibration isolation and near frictionless actuation needed. Additionally, actuation capability is limited to the force capability of the electric motor.
There remains a need for a compact, lightweight, high bandwidth actuator having essentially frictionless performance and relatively large stroke and force capacity while also having the characteristics at high frequency of a very soft passive. The invention described herein is such a device.